1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting a reception acknowledgement for hybrid automatic repeat request (HARQ) in a wireless communication system based on time division duplex (TDD).
2. Related Art
Long term evolution (LTE) based on 3rd generation partnership project (3GPP) technical specification (TS) release 8 is a promising next-generation mobile communication standard.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, a physical channel of the LTE can be classified into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
The PUCCH is an uplink control channel used for transmission of an uplink control signal such as a hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal, a channel quality indicator (CQI), and a scheduling request (SR).
Meanwhile, 3GPP LTE-advanced (A) which is an evolution of 3GPP LTE is under development. Examples of techniques employed in the 3GPP LTE-A include carrier aggregation and multiple input multiple output (MIMO) supporting four or more antenna ports.
The carrier aggregation uses a plurality of component carriers. The component carrier is defined with a center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier is mapped to one cell. When a user equipment receives a service by using a plurality of downlink component carriers, it can be said that the user equipment receives the service from a plurality of serving cells.
A time division duplex (TDD) system uses the same frequency in downlink and uplink cases. Therefore, one or more downlink subframes are associated with an uplink subframe. The ‘association’ implies that transmission/reception in the downlink subframe is associated with transmission/reception in the uplink subframe. For example, when a transport block is received in a plurality of downlink subframes, the user equipment transmits HARQ ACK/NACK for the transport block in the uplink subframe associated with the plurality of downlink subframes.
With the introduction of a plurality of serving cells in the TDD system, a payload of the HARQ ACK/NACK is increased in size. It is important to increase transmission reliability of the HARQ ACK/NACK to guarantee reliability when performing HARQ. However, if the payload of the HARQ ACK/NACK is excessively great, it is difficult to increase a data rate for user traffic.
Accordingly, there is a need for a method capable of decreasing the increased payload of the HARQ ACK/NACK while maintaining transmission reliability of the HARQ ACK/NACK.